In many situations it is necessary to determine the busy-idle status of a telephone line without actually connecting the telephone network across the line. For example, in a situation where a number of telephone lines end in jack connections it is sometimes advantageous to plug the telephone, or other communication equipment, into a jack associated with a known idle line without first audibly sampling each line to determine which line is idle.
Although the solution to the problem, namely, placing a light indicator device across the communication leads, at first appears straightforward, there are many complicating factors which must be taken into account. Of foremost concern when designing a line monitoring device is the fact that the current drain of the device must be low enough so that the switching machine does not falsely detect an off-hook condition. Also, the current drain must be sufficiently low so that any line test equipment placed on the line will not indicate a fault condition.
As an indication of the need for a telephone line status indicating device, the multiple extension home telephone system is a typical example. Since more than one telephone station set may become connected to the same telephone line, it often happens that while one person is communicating from one telephone station set a second person at a second station set, thinking the line is idle, lifts the receiver of the second station set and becomes bridged across the established connection. This is an undesirable situation and one that can be avoided if there is provided at each telephone station set location a visual indication of the status of the associated line.
In situations where it is possible to add additional wiring to telephone installations, U.S. Pat. No. 3,906,168, issued Sept. 16, 1975 to J. R. McEowen, and entitled "Visual Status Indicator Circuit" teaches one possible solution. However, in existing installations it is usually uneconomical to add additional wires between the telephone locations. Thus, a need exists in the art for an economical system of indicating to a subscriber at one telephone station set location the busy-idle status of the associated telephone line in situations where that telephone line is connectable to telephone station sets at different locations without necessitating the use of wires other than those which exist for communication purposes, and without excessive current drain from the switching machine.